plumbing/transport/http: add callback based basic auth

[roboll]

Hey - great work.

I'm using go-git in a Github app which uses dynamic credentials, so having http basic auth backed by a callback function is really useful. Let me know what you think of this.

[codecov]

Codecov Report

Merging #419 into master will decrease coverage by 0.88%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
- Coverage   78.03%   77.14%   -0.89%     
==========================================
  Files         127      124       -3     
  Lines        9186     9024     -162     
==========================================
- Hits         7168     6962     -206     
- Misses       1237     1304      +67     
+ Partials      781      758      -23

Impacted Files	Coverage Δ
plumbing/transport/http/common.go	73.33% <60%> (-2.67%)	⬇️
plumbing/transport/ssh/common.go	0% <0%> (-50.91%)	⬇️
plumbing/transport/ssh/auth_method.go	33.33% <0%> (-24.77%)	⬇️
plumbing/transport/git/common.go	70.73% <0%> (-10.23%)	⬇️
worktree_commit.go	70.49% <0%> (-3.43%)	⬇️
worktree_status.go	69.66% <0%> (-1.71%)	⬇️
storage/filesystem/internal/dotgit/writers.go	75.73% <0%> (-1.48%)	⬇️
worktree.go	64.52% <0%> (-1.42%)	⬇️
remote.go	70.81% <0%> (-0.84%)	⬇️
plumbing/revlist/revlist.go	80.59% <0%> (-0.57%)	⬇️
... and 11 more

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[mcuadros]

this function is completely untested, can you cover it?

[smola]

So far the preferred way to implement this kind of logic is to set the basic auth from the beginning, or using ErrAuthRequired to ask for the password only when required. See example: #350 (comment)

[smola]

There is also a pending PR adding an example about intended usage: #383

[roboll]

hey @smola - I did see the other PR and example usage. I think this code solves an additional use case, which is constantly changing credentials. For example, I'm using this library with credentials that are rotated every hour as part of a Github App.

I could conceivably handle it externally, but this felt like a common enough use case to try to include here. If you'd rather not, I understand.

@mcuadros If you all think you may accept this, then yes, I'll definitely follow up with some test coverage for that function 👍.